Here's how to do it quickly in CS2. I think it'll work in CS as well, but probably nothing earlier.

Select the crop tool and click the "front image" button at the top of the screen. This will make your selection the same aspect ratio (shape) as the whole picture.

Draw a rectangle somewhat smaller than the whole picture, then click and drag outside of the rectangle to rotate. Line it up with the seams in the walls. Hit enter or the checkmark when it looks about right.

If it looks good, continue on. If not, undo and try again.

Make sure you have your rulers on. If they aren't, hit ctrl-R.

Drag a guide from the left ruler and place it at around the center of the base of tree.

Select the entire image (ctrl-A).

Transform the selection (ctrl-T).

While in transform, look at the top of the screen for a button that looks kind of like a sad face. It's to the left of the checkmark and the circle with a line through it. Click that button.

This puts you in warp mode. It allows you to make curved transforms to the image. Click on the star of the tree and drag it to the right until it looks straight. Be careful not to drift downward as it leave a sliver along the top edge (edit: as you can see on this one as it was done too hastily :sad.